Current measuring method and device

ABSTRACT

A method of determining the magnitude of an exceptionally large current which comprises measuring the angle of rotation of polarized light in a magnetic field created by the current with a glass fiber sensor composed of a fluoride glass having a low Verdet constant and a photoelastic coefficient not over about 0.25 (nm/cm)/(kg/cm 2 ).

[0001] This application claims the benefit of priority under 35 U.S.C.§119 from the European Patent Application Number 99400016.4, filed Jan.5, 1999 and U.S. provisional application Ser. No. 60/116,706, filed Jan.22, 1999.

FIELD OF THE INVENTION

[0002] The invention relates broadly to measurement of large currentsand production of devices for that purpose.

BACKGROUND OF THE INVENTION

[0003] Fiber optic, current sensors, based on the Faraday effect, have anumber of advantages for remotely measuring large electrical currents.These include wide dynamic range, fast response, immunity toelectromagnetic interference, small size, and low cost. Consequently, avariety of fiber optic, current sensors have been investigated in recentyears. Mainly, they have employed a single mode optical fiber (SMF) ofclad silica.

[0004] These sensors have not yet reached the stage of practical fielduse due to lack of accuracy and stability. This is mainly due tointrinsic and induced, linear birefringences that distort the Faradayrotation being measured. A particular problem arises from the inabilityof silica fibers to measure accurately large currents, such as surge orfault currents. Such currents are exceptionally large, as much as 180 kAunder some circumstances. They generally occur due to some failure, suchas a short circuit.

[0005] The Faraday effect is a phenomenon by which a linear, polarizedlight will rotate when propagating through a transparent material thatis placed in a magnetic field in parallel to the magnetic field. Thesize of the rotation angle (θ), given in degrees, is defined as

[0006] (1)θ=VHL

[0007] where H is the strength of the magnetic field (A/m), V is theVerdet constant of the material, and L is the path length over which themagnetic field acts (m).

[0008] The magnetic field strength is measured in terms of Amperes (A)times turns (T) per unit length (AT/m) where m is meters). Since valuesare expressed in terms of one turn, this factor is usually implicit,rather than explicit. Hence, the strength is customarily given inamperes (A) or kiloamperes (kA) per unit path length in meters (m).

[0009] The Verdet constant, V, is the angle of rotation divided by themagnetic field strength per unit length. The angle may be expressed inany of the customary units for angle measurement, but degrees are usedhere. Verdet constant values, unless otherwise indicated, are given interms of degrees divided by field strength expressed as (kA×T/m)m.

[0010] The magnitude of the magnetic induction (B) around an infinitestraight conductor is given by the expression:

[0011] (2) B=(μ_(∘)/4π)(2l/a)

[0012] where I is the current, μ∘ is permittivity of free space, and ais the radial distance of the magnetic field from the conductor. Themagnetic field is related to the magnetic induction by the simplerelation:

[0013] (3) B=μ_(∘H.)

[0014] Combining equations 1 through 3 gives a proportional relationbetween the rotation and the current such that:

[0015] (4)θ=VI

[0016] where θ is in degrees, V is the Verdet constant, and I is inkiloamperes (kA). Thus, the sensitivity of a method for measuring thecurrent depends on how accurately the angular rotation can be measured.

[0017] The degree of sensitivity in measuring the angular rotation isinfluenced by another factor; birefringence. Birefringence arisesprimarily from stresses that result from bending, or otherwisedistorting, a fiber in its disposition. The sources of linearbirefringence in single mode fibers include residual stress fromfabrication, bending, contact, and thermal stresses (Yamashita et al.,“Extremely Small Stress-optic Coefficient Glass Single Mode Fibers ForCurrent Sensor”, Optical Fiber Sensors, Sapporo Japan, paper We2-4, page168 (1996) (“Yamashita”).

[0018] The stress-induced birefringence is quantified in terms of acoefficient, called the photoelastic constant (or the photoelasticcoefficient). The photoelastic coefficient (B_(ρ)) may be defined as thecoefficient relating the difference in the refractive indices in thestress direction (n(par)) and in the perpendicular direction (n(per)),to the magnitude of the applied stress:

[0019] (5) n(par)−n(per)=B_(ρ)σ

[0020] It may also be regarded as the phase shift measured in units ofwavelength in nanometers (nm) per path length in centimeters (cm)divided by the stress in kilograms per square centimeter (kg/cm²). Thevalues then are in units of (nm/cm divided by kg/cm²).

[0021] An ideal glass fiber would have a photoelastic coefficient ofzero, thereby nullifying any effect of stress-induced birefringence.However, this has proven difficult to obtain in conjunction with otherdesired properties. Therefore, a near-zero value, e.g., a value within arange of −0.2 to 0.2, has been considered adequate for some purposes.

[0022] In measuring a surge current, it is important to keep the angleof rotation below 90 degrees. With glass fibers having large Verdetconstants, a fault current measurement is apt to create an angle ofrotation greater than 90 degrees. The angle of rotation greater than 90degrees will register the same as an angle of less than 90 degrees. Incontrast, a device having a glass fiber with a low Verdet constant willnot have as great an angle of rotation when measuring a large faultcurrent. Therefore, it will accurately measure such currents.

[0023] It is a purpose of the present invention to provide an improvedmethod and device for measuring large currents, such as surge and faultcurrents.

[0024] Another purpose is to provide a glass that is adapted to use insuch improved method and device.

[0025] A further purpose is to provide a method of producing a glasshaving a near-zero photoelastic coefficient in conjunction with a lowVerdet constant.

[0026] A still further purpose is to provide a method of reducing thephotoelastic coefficient of a glass having a low Verdet constant.

SUMMARY OF THE INVENTION

[0027] The present invention resides in part in a method of reducing thephotoelastic coefficient of a fluoride glass that has a low Verdetconstant at a wavelength suitable for measurement, and that containszirconium fluoride as a primary component of its composition, the methodcomprising the step of incorporating a small amount of lead fluoride inthe glass composition.

[0028] The invention further resides in a method of determining themagnitude of a surge or fault current of up to about 200 kA whichcomprises:

[0029] providing a glass fiber, current sensor, the glass having acomposition composed predominantly of zirconium fluoride and containingup to about 3% lead fluoride, having a low Verdet constant at thewavelength used for measurement, and capable of causing an angularrotation of polarized light less than 0.45° per kA, per pass at thatwavelength,

[0030] passing a current through a conductor to create a magnetic fieldsurrounding the conductor,

[0031] positioning the current sensor within the magnetic field thuscreated,

[0032] propagating polarized light into the glass fiber, current sensor,

[0033] measuring the angle of rotation of the polarized light in theglass fiber sensor, and

[0034] determining the magnitude of the current from the angle ofrotation of the polarized light.

DESCRIPTION OF THE DRAWINGS

[0035] The single FIGURE in the accompanying drawing is a device forcarrying out the method according to the present invention.

DESCRIPTION OF THE INVENTION

[0036] The present invention relates to a method and device fordetermining the magnitude of an exceptionally large current. Themagnitude is determined by measuring the angle of rotation that thecurrent creates in polarized light as the light is transmitted through afiber in a magnetic field. The angle of rotation is less when the glassfrom which the fiber is drawn has a low Verdet constant. In particular,a fiber produced from a fluoride glass of the present invention willhave a Verdet constant that is less than 0.45 degrees/kA. Therefore,when the fiber is exposed to a current up to at least 200 kA inmagnitude, it will register an angle of rotation less than 90 degrees,and will accurately measure the current.

[0037] Reference to a fiber signifies a clad fiber comprisingessentially a fiber core and an outer cladding layer. The fiber core isthe functional member for current measurement. However, it is well knownthat a fiber core requires a cladding of lower refractive index toprevent loss of light from the core during transmission.

[0038] Except for refractive index, it is desirable that the propertiesof a cladding closely match those of a core glass in a clad fiber.Accordingly, it is common practice to use glasses of the samecomposition family for the core and cladding. The cladding glass willhave the same composition as that of the core glass, except modified toimpart a lower refractive index.

[0039] Lead fluoride (PbF₂) is added to a selected fluoride glasscomposition, in exchange for sodium fluoride (NaF), to produce a fibercore. A glass having the same composition, but with the PbF₂ omitted andthe NaF restored, may be used as a cladding glass. Other exchanges, suchas for barium fluoride and/or zinc fluoride, may be made to lower theindex for a cladding glass. For example, NaF, or another alkaline earthmetal fluoride, such as calcium or magnesium fluoride, may be exchangedfor barium or zinc fluoride to provide a lower refractive index. The twoglasses may be melted, and a clad fiber drawn employing the well-knowndouble crucible technique.

[0040]FIG. 1 illustrates an embodiment of the device of the presentinvention. Preferably, a clad fiber 3, as described above, is utilized.However, any glass article, such as a piece of bulk glass (not shown),can be used. Fiber 3 acts as a path for the polarized light. Conductor 4carries the current to produce a magnetic field. Preferably, fiber 3 iswrapped around conductor 4, as shown, to increase the length of thelight path. Also, it is preferable that fiber 3 be insulated from theconductor.

[0041] The device also includes a source of light rays 1, the sourcebeing located such that light rays are directed to an input end of fiber3. Typically, the source of light rays 1 is a laser. A polarizer 2 islocated adjacent to source of light rays 1 such that the light rays arelinearly polarized. An analyzer 5 is located at an output end of fiber3.

[0042] Analyzer 5 derives a rotatory, polarization component produced inproportion to the current flowing through the conductor 4. Also includedis a means 8 for indicating the measured current corresponding to theoutput of analyzer 5. Typically, the means 8 is a light detector 6, anda display device 7. Light detector 6 receives and detects the output ofanalyzer 5. Device 7 receives the output of, and provides a display of,the output of the light detector 6.

[0043] Optionally, the analyzer may be a Wollaston prism, as describedin Yamashita. Then, the light ray output from the fiber is broken intotwo orthogonal polarizations. Means 8 detects the output of each signal,and indicates the measured current corresponding to the output.

[0044] Electrical current, in a normal power station operation, can bedetermined by employing a glass current sensor, preferably in the formof a clad fiber. In order to avoid the effect of birefringence, it isdesirable to employ a glass having a low photoelastic coefficient,preferably zero or near-zero. The Verdet constant of the glass may thenordinarily be as large as possible to enhance the sensitivity of thedetermination. In measuring exceptionally large currents, as explainedearlier, a low Verdet constant is now required. This avoids pushing theangle of rotation of the polarized light beyond 900.

[0045] Heretofore, fused silica has provided the smallest Verdetconstant available in an inorganic glass, the value being 0.10/kA at1150 nm. However, fused silica also has a large photoelasticcoefficient, 3.5 (nm/cm)/(kg/cm²) at 560 nm. This has led to a searchfor a glass of comparable Verdet constant and a low, near-zerophotoelastic coefficient.

[0046] It has been observed that certain fluoride glasses haverelatively small photoelastic coefficients. Further, these glasses mayalso have small Verdet constants. Particular reference is made to afluoride glass known by the acronym ZBLAN. This glass is reported tohave a composition consisting of, in mole percent, 53 ZrF₄, 20 BaF₂, 4LaF₃, 3 AIF₃ and 20 NaF. Measurements on this glass show a desirably lowVerdet constant of 0.22°/kA at 633 nm., and a photoelastic coefficientof 0.34 (nm/cm)/(kg/cm²) at 546 nm.

[0047] In order to measure exceptionally large currents, an even lowerphotoelastic coefficient is desirable. We have found that lead fluoride(PbF₂) can be added to a ZBLAN-type glass composition in an amount up toabout 3 mol %. Preferably, the addition is in substitution for sodiumfluoride (NaF).

[0048] We have further found that such additions result in glasseshaving decreased photoelastic coefficients. Based on these findings,core fibers for present purposes preferably consist essentially of, ascalculated in mol %: 52-56% ZrF₄ 14-24% BaF₂ 3-6% LaF₃ 3-6% AlF₃ 14-22%NaF up to about 3% PbF₂

SPECIFIC EMBODIMENTS

[0049] Three glasses were prepared based on the ZBLAN composition. Intwo of the glass compositions, a small amount of lead fluoride (PbF₂)was substituted for sodium fluoride (NaF). Otherwise, the ZBLANcomposition was unchanged.

[0050] Measurements were made of Verdet coefficient at 633 nm, and at 1150 nm for one glass, and photoelastic coefficient (B) at 546 nm. TABLEI shows the PbF₂ content in mole and weight % and the measured valuesfor Verdet and photoelastic coefficients. TABLE I PbF₂ mole % weight % V(633 nm) V (1150 nm) B (546 nm) 0.0 0.0 0.22 — 0.34 0.7 1.43 0.22 0.120.25 2.0 4.28 0.20 — 0.18

[0051] These data indicate that increasing PbF₂ substitutions willprovide a photoelastic coefficient approaching zero. However, thecompositions become increasingly difficult to melt.

[0052] The glasses shown in TABLE I were prepared by mixing anappropriate batch of fluoride components, placing the batch in acovered, platinum crucible to retain fluorine, and melting at 800° C.for about 40 minutes. The crucible was then uncovered, and the melt washeat treated for 2-3 hours while being covered with gaseous sulfurhexafluoride. The melts were then poured into molds heated at 260° C.and the glasses annealed at that temperature. The annealed glasses wereclear, and test pieces were prepared for measurements as recorded inTABLE I.

We claim:
 1. A method of reducing the photoelastic coefficient of afluoride glass that has a low Verdet constant at a wavelength suitablefor measurement, and that contains zirconium fluoride as a primarycomponent of its composition, the method comprising the step ofincorporating a small amount of lead fluoride in the glass composition.2. The method of claim 1 which comprises incorporating an amount of leadfluoride in the fluoride glass composition that is not over about 3.0mole %.
 3. The method of claim 1 which comprises incorporating the leadfluoride in a fluoride glass composition which is predominantlyzirconium fluoride and contains smaller amounts of barium, lanthanum,aluminum, and sodium fluorides.
 4. The method of claim 3 which comprisesincorporating the lead fluoride in a glass composition consistingessentially of, in mole %: 53% ZrF₄, 20% BaF₂, 4% LaF₃, 3% AIF₃ and 20%NaF.
 5. The method of claim 1 which comprises reducing the photoelasticcoefficient to a value less than about 0.20 (nm/cm)/(kg/cm²).
 6. Amethod of determining the magnitude of an exceptionally large current ofup to about 200 kA which comprises, providing a glass fiber, currentsensor, the core glass of which has a composition composed predominantlyof zirconium fluoride and containing up to about 3 mole % lead fluoride,having a low Verdet constant at the wavelength used for measurement, andcapable of causing an angular rotation of polarized light less than0.45° per kA, per pass at that wavelength, passing a current through aconductor to create a magnetic field surrounding the conductor,positioning the current sensor within the magnetic field thus created,propagating polarized light into the glass fiber, current sensor,measuring the angle of rotation of the polarized light in the glassfiber sensor, and determining the magnitude of the current from theangle of rotation of the polarized light.
 7. The method of claim 6 whichcomprises forming the glass current sensor as a coil through which theconductor extends.
 8. The method of claim 6 which comprises providing aglass fiber sensor that has a photoelastic coefficient not over 0.25(nm/cm)/(kg/cm²).
 9. A glass fiber sensor for measuring an exceptionallylarge current in a magnetic field, the core glass having a fluorideglass composition containing ZrF₄ as a primary component and not over 3mole percent PbF₂ whereby the photoelastic coefficient of the glass isreduced to a value not over 0.25 (nm/cm)/(kg/cm²).
 10. A glass fibersensor in accordance with claim 9 wherein the core glass consistsessentially of, as calculated in mol %: 52-56% ZrF₄ 14-24% BaF₂ 3-6%LaF₃ 3-6% AlF₃ 14-22% NaF up to about 3% PbF₂